b. Rating Chart Selection. It is important when applying tube-rating charts to
use the proper one. For example, if the x-ray machine has full-wave rectification, then
the chart for full-wave rectification must be used since tube capacity varies with the type
of rectification and charts may be available for both. Tube capacity also varies with
single or three- phase generators. So, again, the appropriate chart must be used.
Factors such as those mentioned above need to be determined only once and will
remain constant, unless there is a major modification in the machine or in the x-ray tube
itself. One factor, however, that can vary from exposure to exposure, and has a very
definite effect on tube capacity, is focal spot size. Consequently, the x-ray specialist
must be aware of the size of the focal spot he is using so he can apply the appropriate
tube-rating chart.
(1) Radiographic (tube) rating chart. This chart shows the maximum
exposure factors allowable for a single radiographic exposure. Figure 3-16 shows two
such charts for the same tube. One is for a 2-mm (large) focus and the other is for a
1-mm (small) focus. The factors to be considered on these charts are mA, kVp, and
exposure time; when any two of them are known, the third can be easily found.
(a) For example, with the large focus, using 150 mA and 90 kVp, the
maximum exposure time allowable could be found as follows: Find 150 mA on the left
margin of the chart, and follow the line across the chart until it intersects with the 90-kVp
line. From that point, follow an imaginary line straight down to the exposure time scale.
The maximum exposure time in this case would be 4 1/2 seconds. Anything above that
would exceed the tube's capacity and could damage the tube.
(b) By following the same procedure on the small focus chart, you can
see that the same mA and kVp would allow a maximum exposure time of only 3/4
second. This comparison shows that the load capacity of a tube using the small focus
is considerably less than with the large focus.
(2) Angiographic rating chart. In calculating the maximum number of
exposures in angiography before the tube is allowed to cool, you will need to consult an
angiographic rating chart for that tube. First, find the heat units (HU) for each exposure
as discussed above. Then find the line for that HU on the chart and coordinate this with
the number of exposures per second you are to make. This will show the number of
exposures you can make before cooling the tube.
(3) Cineradiographic rating charts. Cineradiographic rating charts are
arranged in various ways. It is very important to make your calculations carefully using
a chart correct for the tube you have in use.
MD0950
3-22
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